Single-threaded scheduler

Static molecules are now restricted to a linear output context, in a similar way to blocking replies. Reaction schedulers now run on a single dedicated thread; site pools are eliminated. New examples: 8 queens and hierarchical map/reduce. Added a simple facility for deadlock warning, used for FixedPool. Tutorial updated with a new "quick start" guide that avoids requiring too many new concepts. Miscellaneous bug fixes and performance improvements.